It is known to provide a liquid flow control assembly in which a body member, providing at least one passage, and a closure member are relatively movable between a first condition in which the closure member prevents flow of liquid through the or each passage and a second condition in which the body member and the closure member define at least one aperture for the flow of liquid between the body member and the closure member.
Thus, many devices used within a hydraulic system employ means whereby the pressure or flow rates of the system working fluid is controlled by passing the working fluid through a narrow aperture. The control of fluid pressure or flow may form part or the whole of the function of the device.
Commonly, the aperture is formed by an annular gap between a cone and a circular orifice concentric with the axis of the cone and of diameter less than the base diameter of the cone. Variation in the flow area available through the annular gap is commonly achieved by movement of the cone and orifice relative to one another along the axis of the cone. Such an arrangement is often referred to as a "cone and seat", "plug and seat", "poppet valve" "globe valve" assembly.
Alternatively, the narrow aperture hereinbefore mentioned may be formed by the circumferential gap between the end of a cylinder and openings formed in a close fitting sleeve around the cylinder. Variation in the flow area available through the openings in the sleeve may be achieved by relative movement of the sleeve and cylinder along the cylinder axis, such that the cylinder covers a greater or smaller part of the sleeve openings. Such an arrangement is commonly referred to as a "spool and sleeve valve" assembly.
As a further alternative, the narrow aperture hereinbefore mentioned may be formed by the crescent shaped gap between a circular conduit and a circular plate partially blocking off this conduit. Variation in the flow area may be achieved by movement of the circular plate along a line perpendicular to the axis of the conduit. Such an arrangement is commonly referred to as a "gate valve" assembly.
It is known feature of "poppet valve", "spool and sleeve valve" and "gate valve" assemblies that high fluid velocities can occur in the region of the aperture between the body member and the closure member comprising the controlling elements and that this results in low local pressures in the fluid after it has passed through the aperture. Gas and vapour bubbles commonly form in this region, giving rise to cavitation which is frequently in the form of a "plume" of cavitation bubbles emerging from the assembly.
The presence of cavitation within a hydraulic device may result in noise, mechanical vibration, fluid pressure fluctuations, and physical damage. It is therefore desirable that the shapes of the pressure or flow controlling elements within any hydraulic device be such as to reduce as far as possible the tendency for cavitation to occur and to control and contain any cavitation "plume" that may form. It is also desirable that the flow or pressure controlling elements of a hydraulic device be so formed as to reduce the tendency for "instability", or low frequency oscillation, of the controlling elements.